The present study investigates the effect of synthetic diamond nanoparticles as functional modifiers of NiCoP coatings, simultaneously addressing their corrosion resistance and bifunctional HER/OER electrocatalytic performance on Fe-steel substrates. A direct comparison between electroless and electrodeposition routes is presented. The corrosion behaviour of the coatings was evaluated ex situ in 1 M and 6 M KOH electrolytes to assess their stability under both operating and accelerated degradation conditions. The as-prepared and alkali-treated coatings were characterized by potentiodynamic polarization measurements, X-ray diffraction (XRD), field-emission scanning electron microscopy coupled with energy-dispersive spectroscopy (FE-SEM/EDS), and X-ray fluorescence (XRF) analysis. In addition, their bifunctional electrocatalytic performance toward the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) was evaluated. The results demonstrate that synthetic diamond incorporation influences the corrosion behaviour of NiCoP coatings; however, the magnitude and direction of this effect depend strongly on both the deposition route and the electrolyte concentration. Furthermore, co-electrodeposition combined with synthetic diamond nanoparticles emerges as a promising approach for developing PGM-free, diamond-modified NiCoP composite coatings on commercially available Fe-steel substrates, providing an effective balance between coating microstructure, corrosion resistance, and bifunctional electrocatalytic performance.